1. Field of the Invention
This invention relates to a finder suitable for use in a lens shutter camera and the like in which a photographic lens system is provided such that it is independent of a finder lens system.
2. Description of Related Art
Keplerian finders are designed so that a field frame and additional marks are provided adjacent to the intermediate image plane of an objective system and can be observed through an ocular system. Thus, the boundary line of the frame is sharply viewed. Moreover, since a visual field itself can be clearly recognized with little flare, the view of the visual field is very fine. An entrance window, because its location is near an entrance pupil, can be designed to diminish in size, and compactness of the finder can be achieved, depending upon how each of prisms constituting an image erecting system is configured to bend an optical path. For these reasons, Keplerian finders often have been used as finders for high-grade lens shutter cameras, in particular. However, since there is the drawback that dust particles adhering to the components located close to the intermediate image plane are liable to be seen, great care must be exercised at the manufacturing stage of the finder to avoid this difficulty.
Such a Keplerian finder, compared with a conventional Albada finder, has the great advantage that the view of the visual field is favorable. On the other hand, it has the defect that because the entire system of the finder includes an objective system, an image erecting system, and an ocular system, the overall length of the finder is greater. Consequently, when such a finder is mounted in a camera, the thickness of a camera body becomes larger. With a variable magnification finder in particular, a space sufficient for moving lenses in it is required for variable magnification of the finder and consequently entire length of the objective system becomes larger. This is responsible for a further increase in the thickness of the camera body. However, in view of the latest needs of the improvement of a variable magnification ratio and the compactness of the camera, it has become an important matter to reduce the overall length of the finder.
In Keplerian finders, many objective systems which are constructed as zoom lens systems have been invented in the past, and zoom lens systems for varying the magnification of the finder are available in various types. In particular, an objective system composed of three lens units having negative, positive, and positive powers, respectively, is favorably corrected for aberrations and can be compactly designed. Thus, for such objective systems, there are numerous examples of prior art.
Keplerian variable magnification finders having the objective systems mentioned above are known which are disclosed, for example, in Japanese Patent Preliminary Publication Nos. Hei 1-131510, Hei 4-53914, Hei 4-219711, and Hei 6-51201. Any of these finders is such that the objective system is constructed with three zoom lens units with negative, positive, and positive powers so that a first lens unit is fixed and second and third lens units are moved to change the magnification and compensate for the positional shift of an image plane.
The finder disclosed in Hei 1-131510 mentioned above is constructed so that when the magnification is changed from a low magnification position to a high magnification position, the second lens unit of the objective system is moved toward an object and the third lens unit is moved toward an image. Hence, the second lens unit practically bears variable magnification behavior and the third lens unit mainly serves to compensate for the positional shift of the image plane. Four reflecting surfaces are arranged between the intermediate image plane formed by the objective system and the ocular system. As set forth in the prior art examples disclosed after this, however, an arrangement is such that as a result of the back focal distance of the objective system increasing so that reflecting surfaces for forming an erect image can be placed in the objective system, the third lens unit gradually comes to bear the variable magnification behavior and can be moved together with the second lens unit toward the object. In this case, reflecting members constituting reflecting surfaces are such that optical path lengths required vary according to how to bend optical paths. However, if the optical path length is too short, the reflecting surface cannot be constructed, while if it is too long, the entire length of the objective system will be increased. Consequently, it is required that the reflecting member is designed to provide an optimum optical path length.
A finder for effectively fulfilling this requirement is disclosed, for example, in Japanese Patent Preliminary Publication No. Hei 5-164964. This finder is such that the reflecting member is composed of a prism, the entrance surface of which is provided with a negative power to thereby control the back focal distance of the objective system.
Japanese Patent Preliminary Publication No. Hei 6-118303 discloses a finder in which a third lens unit In the objective system is constructed with a prism. This finder is such that the magnification is changed by a second lens unit in the objective system and diopter is corrected by a fourth lens unit disposed close to the intermediate image plane.
Any of the above-mentioned prior art finders, although of a low variable magnification ratio, is considerably great in the entire length of the objective system to hamper the development of compact design of the camera.